1. Field of the Invention
The present invention relates to downhole lubricants and spotting fluids. More particularly, the present invention relates to the use of a downhole drilling compound used as an additive in water-based drilling fluids to lubricate the drill pipe during the drilling process and to free the drill pipe that has become stuck during the drilling process. The present invention further relates to the use of a synthetic hydrocarbon fluid, in particular polyalphaolefin (PAO) liquid containing no more than 0.5% 1-decene monomer, blended in a concentration range of at least 5% by volume with emulsifiers; sometimes referred to as the PAO/emulsifier blend. When introduced at a certain volume downhole, the compound performs as a lubricating agent, used particularly for preventing the drag and torque on drill pipe, and as a spotting fluid, for dislodging stuck drill pipe from the well bore. One distinct advantage of the invention over the previous art in this field, is that the PAO/emulsifier blend is non-toxic to marine life and does not produce a visible sheen when discharged into water bodies.
2. General Background
In the drilling of oil and/or gas wells, a drill bit at the end of a rotating drill string or at the end of a drill motor is used to penetrate through geologic formations. During this operation a drilling mud is circulated through the drill string, out of the bit and is returned to the surface via the annular space between the drill pipe and the formation. The drilling mud, a fluid, cools and lubricates the drill string and drill bit and is designed to counterbalance, through hydrostatic pressure, the encountered formation pressures while providing a washing action to remove the formation cuttings from the wellbore. The drilling mud also forms a friction reducing wall cake between the drill string and the wellbore.
During the drilling of the well, the drill string may demonstrate a tendency to develop unacceptable rotational torque or in the worse case become stuck. At this point, the drill string cannot be raised, lowered and/or rotated. The common factors that can lead to this situation are (1) cuttings or slough buildup in the borehole; (2) an undergauge borehole; (3) irregular borehole development embedding a section of the drill pipe into the drilling mud wall cake; and (4) differential formation pressure unexpectedly encountered.
In the case of differential sticking, the hydrostatic fluid pressure of the drilling mud is greater than the permeable pressure of the exposed formation causing the flow of drilling mud into that area of the formation thus lodging the drill pipe against the formation face. When this occurs, the contact area of the drill pipe and the formation is great enough to cause an increase in rotational torque such that it prevents further movement of the drill pipe without a risk of parting the drill pipe string.
Previous publications concerning methods of preventing drill pipe from sticking and/or freeing stuck drill pipe have discussed the common method of using an oil mud or oil or water based surfactant composition, to reduce friction, permeate drilling mud wall cake, destroy binding wall cake and reduce differential pressure.
There remains a serious need for chemical compositions which can better reduce frictional torque and release stuck drill pipe while demonstrating a low order of toxicity to marine life, more specifically a product to comply with NPDES permit GOM LC.sup.50 test, and not produce a surface sheen on the water body. In contrast to prior art developments which incorporated the use of refined crude oil, diesel, kerosene, mineral oil and most recently low polynuclear aromatic mineral oils (as described in International Pat No. WO 83,102,949), it has been discovered that synthetic oils, and iso-paraffinic oil with no aromatic content, in particular a class of synthetic oils known as polyalphaolefin, demonstrate the required fluid properties and provide the necessary low order of toxicity to comply with the NPDES permit GOM LC.sup.50 to function as the primary composition of downhole fluid additives, more specifically a lubricant and spotting fluid.
Previous developments in this area deal with the application in oil based mud. This invention specifically pertains to the application of these polyalphaolefin compounds as an additive in water based drilling fluids. In contrast to the previous use of synthetic oils in lubricating engines, mills, etc., this invention does not deal with metal to metal, thin film type of lubrication. It is specifically the use of a polyalphaolefin liquid blended with emulsifiers used in a concentration of at least 0.25% by volume introduced as an additive into a water based drilling fluid to lubricate the drill pipe while rotating or pulling past a mud filter cake, hard or soft rock, or casing in a well bore, and to unstick drill pipe that has become differentially stuck.